Remove constraint message being a string

I'm new to Node-RED, so I'm not sure if there is a particular reason that the message needs to be a string.

Though it might have helped me debug a bit if my js object had been visible in the debug node rather than the 'Received message is not a string' error.

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Node-RED seems to handle objects okay:

![image](https://cloud.githubusercontent.com/assets/51385/5038642/ebaf7cd4-6b8a-11e4-9e1e-e92b9711001b.png)